Nuclear magnetic resonance (NMR) is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed NMR active nuclei). Not all atomic types have NMR active nuclei but some common gyromagnetic nuclei include .sup.1 H (protons), .sup.13 C (carbon 13), .sup.19 F and .sup.31 P. When such NMR active nuclei are placed in a strong, uniform and steady magnetic field (a so called "Zeeman field", commonly referred to as an Ho field), they precess about the Ho field direction at a natural resonance frequency known as the Larmor frequency. Accordingly, there is a net magnetization along the Ho field direction, however, no net magnetization transverse to the Ho field. An excitation of the nuclei by a magnetic field transverse to the Ho field direction results in a net rotation of the field magnetization into the XY plane. The excitation of the spins can be accomplished by application of weak RF pulses, continuous wave excitation (CW), adiabatic pulses or DC pulses. Each different type of NMR active nucleus has a characteristic Larmor frequency which is dependent on the strength of the applied magnetic field.
Typically, the material under study is placed within a magnetic coil which generates the uniform Zeeman field. An RF coil for generating a field perpendicular to the Zeeman field direction is commonly used to generate the spin excitation fields. After the spins are excited, the spin excitation field is switched off. The excited spins continue for a certain period after the excitation field is switched off as they "relax" back to equilibrium. As the spins are relaxing, there is a detectable net magnetization in the transverse plane. Since the spin excitation field is off during detection of the NMR field, the same RF coil may then be used to detect the resultant NMR field. The magnitude of the NMR field is a function of the density of the excited nuclei in the volume under observation, commonly called spin density.
As discussed above, the resonant frequency of a nucleus is a direct function of the strength of the Ho magnetic field at that nucleus. Accordingly, if the net magnetic field in the volume of interest is spatially variant (e.g., a magnetic field gradient) as opposed to uniform, the resonant frequency at different spatial locations in the volume under observation will differ. Since the resonant frequency of each nucleus will depend on the strength of the field at the location of that nucleus, the frequency spectrum of the induced NMR conveys information as to the relative concentration of the observed nuclei at different spatial locations in the volume under observation. The variation in Larmor frequency from one type of nucleus to another is generally on the order of tens of MHz. Accordingly, NMR of different nuclei type are easily distinguished and one may easily detect NMR spin density of only one type of nuclei during any given observation.
A Fourier transformation of the detected NMR signal from a sample volume in a linearly gradient field provides direct information about the spatial distribution of the NMR active nuclei along the gradient.
Nuclear magnetic resonance imaging has found significant application in the medical field, and in particular, in observing the human body. Reference is made to Morris, Peter G., Nuclear Magnetic Resonance Imaging in Medicine and Biology, Clarendon Press, Oxford, 1986, for a detailed description of the use of nuclear magnetic resonance imaging in the medical and biological fields. That publication is incorporated herein by reference.
As stated, NMR imaging has traditionally been accomplished by surrounding the object under observation with magnetic coils for generating the Zeeman, RF, and gradient magnetic fields. However, a variety of applications exist where NMR imaging is desirable but physical constraints exist on the placement of the coils relative to the object to be studied. In some such situations, it is useful to be able to generate the magnetic fields for NMR imaging from coils positioned to one side of the object under observation. For example, in geological applications, i.e., where the volume under observation is underground, it is necessary to induce magnetic fields underground. However, there is a planar boundary, the surface of the ground, which separates the volume to be observed and the region where the coils may be placed.
German patent DE 3690746 C2 discloses an NMR imaging apparatus for "visualizing" subterranean water. The apparatus is an NMR hydroscope in which spatial resolution is given only by the spatial extent of the RF field. Accordingly, in order to obtain spatial resolution, the entire apparatus must be moved. Detection of the NMR signals is made by a loop placed on the earth's surface. Information about the depth of the received signal is calculated from the change in signal amplitude and time characteristics. However, information about points shifted in the horizontal plane can only be obtained by moving the entire apparatus horizontally on the surface. The apparatus has no resolution in the horizontal plane and only limited resolution along the vertical axis.
Accordingly, it is an object of the present invention to provide a method and apparatus for performing nuclear magnetic resonance imaging of a volume of observation from one side of the sample volume.
It is another object of the present invention to provide a method and apparatus for performing nuclear magnetic resonance imaging in geological applications.
It is a further object of the present invention to provide a method and apparatus for performing nuclear magnetic resonance imaging of the earth removed to one side of the apparatus.
It is yet another object of the present invention to provide a unique magnetic coil configuration for inducing the necessary magnetic fields for high resolution NMR imaging to one side of the coil arrangement.
It is yet a further object of the present invention to provide a high resolution NMR imaging technique that can be used to map out large volumes without the need to move the magnetic coils.
It is a further object of the present invention to provide a nuclear magnetic resonance imaging technique which can detect nuclear magnetic resonances over 20 meters from the receiver coils.
It is another object of the present invention to provide a high speed method and apparatus for nuclear magnetic resonance imaging from one side of the volume under observation.
It is yet a further object of the present invention to provide a method and apparatus for nuclear magnetic resonance imaging in the earth wherein a high resolution image of the volume under observation can be obtained without movement of the magnetic coils.